Field of Invention
The invention relates to a metal oxide nanoparticle material, and more particularly, to a zirconia nanoparticle and preparation method thereof.
Description of Related Art
Zirconia is widely applied in refractory materials, ceramic appliances and fuel cells for its well heat tolerance and ability of oxygen conductance. Further, zirconia particles with high specific surface area are frequently used as solid catalyst or catalyst support in petrochemical industry. In these applications, using crystalline zirconia nanoparticles can increase the specific surface area and reduce the sintering temperature.
Furthermore, the refractive index and the hardness of zirconia are much higher than that of silicone oxide. Composite of zirconia nanoparticles and organic resins leads to a high refractive index and hardness while keeping a good optical transmittance. The refractive index of the composite is decided by the loading and the crystallinity of the zirconia nanoparticles, while the optical transmittance of the composite depends on the particle size and the aggregation state of the zirconia nanoparticles. For example, adding crystalline zirconia nanoparticles can significantly increase the refractive index of the composite, while the increase will be limited when adding amorphous zirconia nanoparticles. Also, the optical transmittance of the composite is better when adding small and aggregation-free crystalline zirconia nanoparticles, but reduced if the zirconia nanoparticles are large or aggregated. Further, the loading capacity of the zirconia nanoparticles in the organic resins depends on the aggregation state and the aspect ratio of the particles. The loading capacity decreases with increasing particle aggregation, and also with increasing particle aspect ratio.
There are three crystalline structures for zirconia, the monoclinic, the tetragonal and the cubic phases. Generally, the zirconia is in monoclinic phase at room temperature. However, due to the high surface energy, it is possible to keep the crystal in cubic or tetragonal phase even at room temperature if the size of crystalline zirconia nanoparticles is less than 10 nm. With the increase of temperature, the zirconia is subjected to phase transformation from the monoclinic phase to the tetragonal phase at 1170° C., and from the tetragonal phase to the cubic phase at 2370° C. The zirconia transforms from the cubic phase back to the monoclinic phase when cooling down to the room temperature. Since the monoclinic phase has a lower density than that of the high-temperature phases, volume expansion occurs during the cooling and introduces enormous stress resulting in the formation of cracks in the zirconia body. To avoid the phase transformation, calcium, magnesium, strontium, yttrium and related elements are usually added to stabilize the tetragonal or cubic phase zirconia upon decreasing temperature. The aspect ratio of the cubic and the tetragonal phase are usually lower than that of the monoclinic phase. When adding zirconia nanocrystals into an organic resin, the addition of cubic or tetragonal phase has a lower impact on the viscosity of the organic resins. In comparison, the viscosity of the composite quickly increases when adding nanoparticles of monoclinic zirconia having a higher aspect ratio. This significantly limits the amount of zirconia nanoparticles that can be added into the organic resins. Because cubic phase zirconia nanoparticles have the highest symmetry and lowest aspect ratio among the phases, it is desirable to prepare small, non-aggregated, highly-crystalline cubic-phase zirconia nanoparticles.
The zirconia nanoparticles may be fabricated by physical or chemical methods. Since the chemical method is much simpler and cheaper, it has been the favorable method for preparing zirconia nanoparticles. However, due to particle aggregation and insufficient crystallinity, it is difficult to obtain non-aggregated and highly-crystalline zirconia nanoparticles with monomodal size distribution. Further, general methods employed to prepare zirconia nanoparticles usually produce a mixed phases of monoclinic, tetragonal and cubic phases. For example, U.S. Pat. No. 6,376,590 describes the preparation of the zirconia sols without adding a crystal phase stabilizer. The zirconia nanoparticles have an average primary particle size about 20 nm, a crystallinity index of about 70%, and are a combination of cubic and tetragonal phases. TW Patent No. 1401287 describes a transparent dispersion having zirconia nanoparticles of tetragonal phase.
When the zirconia nanoparticles produced by the chemical method are applied to prepare ceramic slurry or to compound with an organic resin, the particle surface must be modified for the dispersion in water or organic solvent. Organic surfactants, chelating agents or surface modifiers are applied on the surface of the zirconia nanoparticles to prevent them from aggregation, so that a stable dispersion or sol in the target solvent can be achieved. Many of the chemical methods for producing zirconia nanoparticles have been done in the presence of organics. The organics may be the surfactant, the chelating agents or even the solvent. It may also come from the reaction media or the zirconium precursor. This produced a large quantity of organic contaminated waste after recovering the product.
If the ligand on the surface of the zirconia nanoparticles does not match the solvent in the target application, a secondary process is required to remove the ligand, and replacing it with another more appropriate one. However, replacing a strong ligand may be both time- and energy-consuming, which significantly deteriorates the efficiency of the process.
To assure their dispersion, the zirconia nanoparticles in the market are generally stored in the form of a sol or a suspension. After the removal of the solvent, the zirconia nanoparticles usually aggregate and fail to be dispersed back into the original solvent. Therefore, it is required to store and transport the zirconia nanoparticles in the sol form.